A new scheme for rapid epitope mapping of the recognition sequence of antigens, receptor lingands, etc. has been devised, involving four steps. Step 1. Rapid digestion of a protein with one or several proteases. Step 2. Immunoprecipitation of the fragments with, for example, a monoclonal antibody, receptor, etc. against the protein antigen. Step 3. Matrix-assisted laser desorption mass spectrometric determination of the peptide(s) that associate with the antibody, receptor, etc. Step 4. Determination of the precise linear epitope with synthetic peptide ladders. Affinity mass spectrometry was used to determine epitopes for Anti-melittin, anti-GLP- 17-3 7 and antihbFGF monoclonal antibodies. A paper describing the technique was published in Proc Natl Acad Sciences USA 93 (1996) 4020-4024